If you can afford a proper flower cooler then that is the best thing to use especially If your shop is on the sunny side of the street. I would only get the drinks cooler if its really cheap or free. You wont be able to fit all your flowers in a drinks cooler and I cant remember what the correct temperature is 5 - 8 degrees I think. The amount of money you save on having a flower cooler will depend on how hot your shop gets, good stock rotation and how fast you can get them out the door.
Hope that helps you